Polystyrene having high degree of expandability, and formulation having a highly-expandable polymer therein

ABSTRACT

A polystyrene polymer as well as an expandable polystyrene formulation comprises a polystyrene polymer having a particular molecular weight distribution. The polymer itself exhibits a polydispersity of from about 1 to less tha 2.0, a weight average molecular weight of greater than about 200,000 to about 300,000, and an M z  :M n  of from about 2 to less than about 3.0. Furthermore, the polystyrene polymer is branched to from 0 to less than 5 weight percent. 
     The expandable polystyrene formulation comprises a polystyrene polymer exhibiting a polydispersity of from about 1.0 to less than 2.5, a weight average molecular weight of from greater than about 180,000 to about 300,000, an M z  :M n  of from about 2 to about 4.5. The polystyrene polymer is present in an amount of from about 94.5 weight percent to about 98 weight percent, based on the total weight of the formulation, and the polystyrene polymer is branched to from 0 to less than 5 weight percent. The formulation further comprises a blowing agent, wherein the blowing agent is present in an amount of from about 2 weight percent to less than 5.5 weight percent, wherein the blowing agent is a hydrocarbon which is gaseous or liquid at standard temperature and pressure, does not dissolve the styrene polymer, and boils below the softening point of the polymer.

This is a division of application Ser. No. 07/617,711 filed Nov. 26,1990.

BACKGROUND OF THE INVENTION

The present invention pertains to a polystyrene polymer exhibiting a newset of characteristics. These characteristics permit a high degree ofexpandability if the polymer is used in admixture with a blowing agent.The present invention also pertains to a formulation of thehighly-expandable polymer in combination with a low level of blowingagent. The high degree of expandibility of the polymer of the presentinvention in turn permits the use of a low level of blowing agenttherewith, resulting in the ability to utilize the formulation of thepresent invention in the production of an expanded polystyrene productof low density (e.g. 0.8 to 1.1 lb./cu.ft.) with blowing agent presentin an amount of from just 2 to 5.5 weight percent, based upon the totalweight of the formulation.

Typical commercial production of expanded polystyrene products hasutilized only "one pass" expansion processes. ["One pass" prosesses arethose which pre-expand styrene particulates only once before the moldingoperation.]These one-pass expansion processes required the use of ablowing agent in an amount of from about 5 weight percent to about 8weight percent for the production of expanded polystyrene productshaving a density of from about 0.9 to about 1.1 lb./cu.ft.

In recent years volatile organic compound (VOC) air emissions have comeunder increasing scrutiny by the EPA, state, and local air qualityboards as mandated by the Clean Air Act of 1977. Because hydrocarbonemissions have been shown to contribute to photochemical smog, theexpanded polystyrene industry which uses pentane as a blowing agent hascome under pressure to limit its use and/or emissions of pentane.

The inventors of the present invention unexpectedly discovered ahighly-expandable polymer which they have characterized. The inventorsalso discovered a formulation (comprising the highly-expandable polymerin conjunction with a low level of blowing agent) which can be used toproduce an expanded polystyrene product having a density of from about0.8 lb./cu.ft. to about 1.1 lb./cu.ft. when the formulation (existing inthe form of solid particles) is expanded in from 2 to 5 expansion steps(i.e. a "multipass" process). As a result, the polymer (and formulationutilizing same) provides the heretofore unavailable advantage ofutilizing a very low level of blowing agent (most preferably about 3.5weight percent, based on the total formulation weight) which therebyresults in the release of a heretofore unachievable low level of blowingagent into the environment during the expansion process.

Since the early months of 1990, the inventors' polymer and formulationhas enjoyed a high level of commercial success, with sales of at least 3million pounds thereof, which formulation has the highly-expandablepolymer present in an amount of about 96 weight percent, based on thetotal weight of product. Thus there has been a high level of commercialsuccess of both the polymer as well as the formulation utilizing thepolymer.

The inventors of the present invention are aware of several related artdocuments and products which are discussed below. First, U.S. Pat. No.4,520,135 and a divisional application thereon (which issued as U.S.Pat. No. 4,525,484) describes a polystyrene particles containing blowingagent and having improved expandability based on polystyrene having aweight-average mean molecular weight of not more than from to 180,000and a molecular weight distribution curve in which the high molecularweight side of the curve drops more steeply. These expandablepolystyrene particles were produced by polymerizing styrene in aqueoussuspension in the presence of blowing agents and of chain transferagents or styrene oligomers.

In contrast to the '135 and '484 patents, the polymer of the presentinvention as well as the polymer within the formulation of the presentinvention have both: (1) a substantially lower polydispersity; as wellas (2) a substantially higher weight average molecular weight, incomparison with the polymers described in the '135 and '484 patents. Ithas surprisingly been found that even though the weight averagemolecular weight of the polymer of the present invention is higher thanthe weight average molecular weight of the '135 and '484 patents, thepolymer of the present invention has a very desirable high degree ofexpandability and moldability.

Other related documents include: U.S. Pat. No. 2,884,386; U.S. Pat. No.4,839,396; U.S. Pat. No. 4,485,193; U.S. Pat. No. 3,639,551; U.S. Pat.No. 3,631,133; U.S. Pat. No. 3,589,769, U.S. Pat. No. 3,126,432, U.S.Pat. No. 3,056,753; and U.S. Pat. No. 4,721,588. None of these patentswarrants any detailed discussion since none discloses subject matterwhich is very close to the polymer and formulation of the presentinvention.

For several years BASF Corporation has been involved in the manufactureand sale of a number of expandable polystyrene formulations havingapproximately 6 weight percent pentane therein. Typically theseformulations contained a polymer having a polydispersity of 2.2, aweight average molecular weight of about 190,000, and an M_(z) :M_(n) ofabout 3.5. In stark contrast, the product of the present invention has apolydispersity of from 1 to less than 2, a weight average molecularweight of from about 200,000 to about 300,000, and an M_(z) :M_(n) offrom about 2 to less than 3.

One polymer which has been commercialized for several years has apolydispersity of about 1.9, a weight average molecular weight of about190,000, and furthermore, upon analysis, yielded an M_(z) :M_(n) of3.04. Furthermore, this polymer was produced only in formulationsbearing blowing agent in an amount of about 6 weight percent. Incontrast, the polymer of the present invention has a combination ofcharacteristics (polydispersity, weight average molecular weight, andM_(z) :M_(n)), which differs from the aforementioned commerciallyavailable polymer. Furthermore, the formulation of the present inventionutilizes blowing agent in an amount of only from about 2 weight percentto about 5.5 weight percent.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a polystyrene polymer exhibiting (i.e.having) three characteristics. First, the polystyrene polymer exhibits apolydispersity of from about 1 to less than 2.0. Second, the polystyrenepolymer also exhibits a weight average molecular weight of from greaterthan about 200,000 to about 300,000. Third, the polystyrene polymerexhibits an M_(z) :M_(n) of from about 2 to less than about 3.0. It isbelieved that these three criteria are so descriptive of the polymerthat together they define only that particular polymer which theinventors have invented. Furthermore, the Comparative Examples hereinshould be noted as providing data regarding several commercialprocesses, and it should be particularly noted that among all of thesepolystyrene polymers, there is none which has all of the characteristicsof the polymer of the present invention.

The present invention also relates to an expandable polystyreneformulation. This formulation comprises both a polystyrene polymer aswell as a blowing agent. The polystyrene polymer (in the formulation)exhibits three characteristics and the blowing agent is present at alevel (of from about 2 weight percent to about 5.5 weight percent, basedon the total weight of the formulation) which has heretofore not beencommercially feasible. The first characteristic of this polystyrenepolymer (present in the formulation) is a polydispersity of from about1.0 to less than 2.5. The second characteristic of this polystyrenepolymer is a weight average molecular weight of from greater than about180,000 to about 300,000. The third characteristic of this polystyrenepolymer is an M_(z) :M_(n) of from about 2 to about 4.5. Furthermore,this polystyrene polymer is present (in the formulation) in an amount offrom about 94.5 weight percent to about 98 weight percent, based on thetotal weight of the formulation.

It is an object of the present invention to provide a polystyrenepolymer having a high degree of expandability.

It is a further object of the present invention to provide a anexpandable polystyrene formulation which may be used to produce expandedpolystyrene products.

It is a further object of the present invention to provide a anexpandable polystyrene formulation which may be used to produce expandedpolystyrene products while using less blowing agent in comparison withprior commercial processes for making expandable pblystyrene products.

It is a further object of the present invention to provide an expandablepolystyrene formulation which incorporates therein a polystyrene polymerhaving a high degree of expandability.

It is a further object of the present invention to provide both apolystyrene polymer and an expandable polystyrene formulation which canbe used to make expanded polystyrene products via processes havingdecreased emissions of volatile organic compounds during storage,preexpansion, postexpansion (but premolding) ageing, and processing.

It is a further object of the present invention to provide both apolystyrene polymer as well as an expandable polystyrene formulationwhich permit processes for making expanded polystyrene products in whicha decreased amount of blowing agent is used.

It is a further object of the present invention to provide a polystyrenepolymer as well as an expandable polystyrene formulation which can beutilized in expansion processes in which there is less blowing agentemitted either to the environment or to pollution abatement equipment.

It is a further object of the present invention to provide an expandablepolystyrene formulation permitting a greater ratio of resin to blowingagent, so that more resin is present per pound of formulation.

It is a further object of the present invention to enable the productionof expanded polystyrene products using decreased molding cycle times, aswell as decreased shrinkage upon molding, as well as decreased agingtimes between expansion steps.

It is a further object of the present invention to enable the productionof an expanded polystyrene product having decreased susceptibility todamage during processing.

It is a further object of the present invention to enable the productionof an expanded polystyrene beads having increased shelf life before themolding step due to a lower rate of loss of blowing agent therefrom.

It is a further object of the present invention to enable a process formaking expanded polystyrene products in which there is decreasedsensitivity to steam during the expansion and molding steps, therebypermitting a "broader molding range" process with respect to the use ofsteam in the preexpansion and molding steps.

It is further object of the present invention to provide a formulationwhich incorporates therein a polystyrene polymer which when preexpandedhas a high degree of moldability.

Each of the above objects can further be understood as providing arespective advantage to the polymer and/or formulation of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polystyrene polymer of the present invention exhibits threecharacteristics: (1) a polydispersity within a given range; (2) a weightaverage molecular weight within a given range; and (3) an M_(z) :M_(n)within a given range. Furthermore, both the polymer and the formulationof the present invention are herein defined in terms of weight averagemolecular weight (M_(w)), number average molecular weight (M_(n)),"z-average" molecular weight (M_(z)). The number average molecularweight is the arithmetic mean value obtained by diving the sum of themolecular weight by the number of molecules. The weight averagemolecular weight is the second power average molecular weight in thepolydisperse polymer. The z-average molecular weight molecular weight isthe third power average molecular weight in the polydisperse polymer.More extensive and descriptive definitions of these various molecularweights were described by Billmeyer, F. W., Jr., Textbook of PolymerScience, 2nd Ed., 1971,Wiley-Interscience, N.Y., N.Y., pp 6, 66, 78, 92.

Polystyrene particles produced according to the invention differ (at aminimum) from those of the prior art with respect of their molecularweight and their molecular weight distribution, as is determined usingthe above-described parameters. The molecular weight distribution curveis determined by gel permeation chromatography. This method is describedin detail in G. Glockler, Polymercharakterisierung, ChromatographischeMethoden, volume 17, published by Huthig, Heidelberg 1982.

The first of these characteristics, i.e. polydispersity, is determinedby analyzing the molecular weight distribution curve for the reactionproduct of the polymerization. Polydispersity is calculated by dividingthe weight average molecular weight by the number average molecularweight. Thus, polydispersity (PD) is a measure of the breadth of thepolymer molecular weight distribution. The polystyrene polymer of thepresent invention generally exhibits a polydispersity of from about 1 toless than 2.0. Preferably, the polystyrene polymer of the presentinvention exhibits a polydispersity of from about 1.5 to less than about2, and most preferably the polystyrene polymer of the present inventionexhibits a polydispersity of from about 1.7 to about 1.98. Example 7(e.g. see the Description of Analytic Procedure, infra) describes themethod of analysis of the polymeric reaction product, this methodproviding the means for determination of weight average molecularweight, number average molecular weight, and z-average molecular weight.Thus this analytical procedure provides the data from which one may thencalculate polydispersity, weight average molecular weight, and the M_(z):M_(n) ratio.

The second characteristic which the polystyrene polymer exhibits (i.e.the weight average molecular weight) is, in general, from greater thanabout 200,000 to about 300,000. Preferably, the polystyrene polymer ofthe present invention has a weight average molecular weight of fromgreater than 200,000 to about 250,000. Most preferably, the polystyrenepolymer of the present invention has a weight average molecular weightof from greater than 200,000 to about 220,000. As with polydispersity,the weight average molecular is determined by the analysis provided inExample 7, infra.

The third characteristic is the ratio of M_(z) :M_(n). This ratio isrelated to the steepness of slope of the upper end of the moleculardistribution curve. In general, the polystyrene polymer of the presentinvention exhibits an M_(z) :M_(n) ratio of from about to 2 to less than3.0. Preferably, the polystyrene polymer of the present inventionexhibits an M_(z) :M_(n) ratio of from about 2.5 to less than 3.0, andmost preferably the polystyrene polymer exhibit an M_(z) :M_(n) ratio offrom about 2.7 to less than 3.0. As with polysidpersity and weightaverage molecular weight, the M_(z) :M_(n) ratio can be calculated basedupon the analytic results obtained from the procedure of Example 7. Thisprocedure, of course, results in obtaining a molecular weightdistribution curve. The value for weight average molecular weight,number average molecular weight, and "z-average molecular weight" can bedetermined. These values permit the calculation of polydispersity aswell as M_(z) :M_(n) ratio.

The polystyrene polymer of the present invention is a substantiallylinear polymer, i.e. is a substantially unbranched polymer. In generalthe polystyrene polymer of the present invention has a degree ofbranching of from 0 to less than 5 weight percent. The phrase ". . .branched to from 0 to less than 5 weight percent . . . " is hereindefined as referring to a polymeric chain in which at least 95 percentof the molecular weight of the polymer resides in that portion of themolecule which constitutes the linear chain. For purposes of calculatingthe weight percent of the polymer which resides in branches (as opposedto the linear portion of the polymer molecule), carbon atoms which arenot part of the main polymeric chain are considered to be located onbranches, and any atoms which are attached to the branch carbon atomsare likewise considered to be located on the branch portion of thepolymer molecule. Non-carbon atoms which are bonded to a carbon atom ofthe linear polymer backbone are considered substituents, rather thanbranches. However, if a substituent atom is bonded directly orindirectly to a second carbon atom wherein the second carbon atom is notpart of the linear polymer backbone, the substituent as well as anyatoms attached thereto (which are not part of the polymer backbone) areconsidered to be a branch. The polystyrene polymer of the presentinvention is preferably branched to from 0 to less than 2 weightpercent. Most preferably the polystyrene polymer of the presentinvention is branched to from 0 to less than 1 weight percent.

The polystyrene of the present invention is preferably a substantiallyhomopolymeric polystyrene polymer. That is, the polystyrene polymer ofthe present invention is preferably derived for a single monomer, thatmonomer being styrene. The phrase "substantially homopolymericpolystyrene polymer" is herein defined as a polymer in which at least 99percent of the monomeric units (which reacted to form the polystyrene)were styrene. Preferably, at least 99.9 percent of the monomeric unitswhich are reacted to form the polystyrene polymer are styrene monomers.

Preferably the polystyrene polymer of the present invention is asubstantially unsubstituted polystyrene polymer. The phrase"substantially unsubstituted polystyrene polymer" is herein defined as apolystyrene polymer having a carbon backbone and branches in which lessthan 2 percent of the available sites for substitution have atoms otherthan hydrogen thereon. Still more preferably, the degree of substitutionis less than 0.5 percent, based on the total number of positions forsubstitution available on the polymer.

A preferred polystyrene polymer of the present invention exhibits: (1) apolydispersity of from about 1.5 to less than 2.0; (2) a weight averagemolecular weight of from greater than to about 200,000 to about 250,000;and (3) an M_(z) :M_(n) of from about to 2.5 to less than about 3.0.Furthermore, this preferred polystyrene polymer is branched to from 0 toless than 2 weight percent. Finally, this preferred polystyrene polymeris a substantially homopolymeric, unsubstituted polystyrene polymer.

A still more preferred polystyrene polymer of the present inventionexhibits: (1) a polydispersity of from about 1.7 to about 1.98; (2) aweight average molecular weight of greater than about 200,000 to about220,000; and (3) an M_(z) :M_(n) of from about 2.7 to less than about3.0. Furthermore, this still more preferred polystyrene polymer isbranched to from 0 to less than 1 weight percent. Finally, this stillmore preferred polystyrene polymer is a substantially homopolymeric,unsubstituted polystyrene polymer.

The present invention also relates to an expandable polystyreneformulation, i.e. a formulation from which expanded polystyrene productscan be produced. In general, this formulation comprises both a specificpolystyrene polymer as well as a blowing agent, wherein the blowingagent is present at a relatively low level. The polystyrene polymerutilized in the formulation of the present invention is herein definedwith respect to three characteristics: polydispersity, weight averagemolecular weight, and M₂ :M_(n), wherein the polystyrene polymer isbranched to from 0 to less than 5 weight percent. Each of the abovethree characteristics are discussed in more detailed below.

The polydispersity of polystyrene polymer utilized in the formulationis, in general, a polydispersity of from about 1.0 to less than 2.5.Preferably the polydispersity of the polymer is from about 1.0 to lessthan 2.0, and still more preferably the polydispersity is from about 1.5to less than 2.0, and most preferably the polydispersity is from about1.7 to about 1.98. The term polydispersity, as used regarding theformulation of the present invention, has the same general meaningdescribed above (i.e. with respect to the polymer of the presentinvention), and the same general analytical methods of determinationapply thereto (as described above with respect to the polymer of thepresent invention).

In general the weight average molecular weight of the polystyrenepolymer within the formulation of the present invention is a molecularweight of from greater than about 180,000 to about 300,000. Preferablythe polymer in the formulation has a weight average molecular weight offrom greater than about 190,000 to about 250,000. Most preferably, thepolystyrene polymer within the formulation has a weight averagemolecular weight of from about 200,000 to about 220,000. As statedabove, the weight average molecular weight is determined via theanalysis described in Example 7.

In general, the polystyrene polymer in the formulation of the presentinvention exhibits a M_(z) :M_(n) ratio of from about 2 to about 4.5,preferably from about 2.5 to about 3.3. Most preferably, the polystyrenepolymer exhibits an M_(z) :M_(n) ratio of from about 2.7 to about 3.0.

In general, the polystyrene polymer in the formulation of the presentinvention is present in an amount of from about 94.5 weight percent toabout 98 weight percent based on the total weight of formulation.Preferably the polystyrene polymer is present in an amount of from about94.6 to about 97.5 weight percent, and most preferably the polystyrenepolymer is present in an amount of about 96 weight percent, based on thetotal weight of the formulation.

In general, the polystyrene polymer in the formulation of the presentinvention is branched to from 0 to less than 5 weight percent.Preferably the polymer is branched to from 0 to about 2 weight percent,and most preferably the polymer is branched to from 0 to about 1 weightpercent. Example 18 (infra) describes how to determine the degree ofbranching of a polymer.

The polystyrene polymer in the formulation of the present invention ispreferably a substantially homopolymeric polymer. Furthermore, thepolystyrene polymer in the formulation of the present invention ispreferably a substantially unsubstituted polystyrene polymer. Withrespect to the polymer utilized in the formulation of the presentinvention, the phrases "substantially homopolymeric" and "substantiallyunsubstituted" have the definitions provided hereinabove with respect tothe polymer of the present invention.

The expandable polystyrene formulation of the present invention furthercomprises a blowing agent. In general the blowing agent is present inthe formulation in an amount of from about 2 weight percent to about 5.5weight percent, based on the weight of the total formulation.Preferable, however, the blowing agent is present in the formulation inan amount of from about 2.5 weight percent to about 4.4 weight percent,based on the total weight of the formulation. Still more preferably, theblowing agent is present in the formulation in an amount of from about 3weight percent to about 4 weight percent, based on the total weight ofthe formulation. Most preferably, the blowing agent is present in theformulation in an amount of about 3.5 weight percent, based on the totalweight of the formulation.

Any one or more of a wide variety of blowing agents can be utilized inthe formulation of the present invention. In general, these blowingagents are hydrocarbons which are gaseous or liquid under normalconditions, do not dissolve the styrene polymer, and boil below thesoften point of the polymer. In general, blowing agents which can beused in the formulation of the present invention comprise at least onemember selected form the group consisting of:

pentane, cyclopentane, neopentane, isopentane, pentane petroleumdistillate fractions, propane, butane, isobutane, hexane,2,2-dimethylbutane, 2,3-dimethylbutane, 2-methyl pentane, 3-methylpentane, methylcyclopentane, cyclohexane, methylcyclohexane, heptane,propylene, butylene, isobutylene, mixtures of one or more aliphatichydrocarbons having a molecular weight of at least 42 and a boilingpoint not higher than 95° C. at 760 millimeters absolute pressure,water, carbon dioxide, ammonium carbonate, and azo compounds that aredecomposable to form a gas at a heat-plastifying temperature to whichthe polymer is brought.

Preferably the blowing agent in the formulation of the present inventionis at least one member selected from the group consisting of:

pentane, cyclopentane, neopentane, isopentane, pentane petroleumdistillate fractions, propane, butane, isobutane, hexane, 2-methylpentane, 3-methyl pentane, methylcyclopentane, cyclohexane, heptane,propylene, butylene, water, carbon dioxide, ammonium carbonate, andmixtures of aliphatic hydrocarbons having a molecular weight of at least42 and a boiling point not higher than 95° C. at 760 millimetersabsolute pressure.

Still more preferably the blowing agent to be utilized in theformulation of the present invention is at least one member selectedfrom the group consisting of:

cyclopentane, neopentane, isopentane, pentane petroleum distillatefractions, propane, butane, isobutane, hexane, 2-methyl pentane,3-methyl pentane, methylcyclopentane, cyclohexane, heptane, propylene,1-butylene, 2-butylene, and mixtures of at least two aliphatichydrocarbons having a molecular weight of at least 42 and a boilingpoint of not higher than 95° C. at 760 millimeters absolute pressure.

The blowing agent utilized in the formulation of the present inventionis still more preferably at least one member selected from the groupconsisting of:

pentane, cyclopentane, neopentane, isopentane, pentane petroleumdistillate fractions, propane, butane, isobutane, hexane, cyclohexane,heptane, and methylcyclopentane.

Most preferably the blowing agent is at least one member selected fromthe group consisting of n-pentane and isomers thereof.

Preferably the formulation of the present invention further comprises achain transfer agent. Chain transfer agents having a transfer constant(as defined in Vollmert, Grundriss der Makromolekularen Chemie,published by Springer 1962, pages 52 and 71, which is herebyincorporated by reference) of from 0.1 to 50, preferably from 1 to 30,are used. Examples of suitable chain transfer agents are:

    ______________________________________                                        n-Dodecyl mercaptan    (K = 19)                                               tert.-Dodecyl mercaptan                                                                              (K = 3)                                                n-Butyl mercaptan      (K = 22)                                               tert.-Butyl mercaptan  (K = 3.6)                                              Carbon tetrabromide    (K = 2.2)                                              Pentaphenylethane      (K = 2.0)                                              ______________________________________                                    

Preferably but optionally the formulation of the present inventionfurther comprises a flame retardant. In general, the flame retardant isan organic bromine or chlorine flame retardant compound present in anamount of from about 0.2 to about 2 weight percent, based on the weightof the total formulation. More preferably the formulation comprises abrominated hydrocarbon flame retardant in an amount fo from about 0.5 toabout 1.5 weight percent, based on the weight of the total formulation.Still more preferably the formulation comprises a flame retardant whichis at least one member selected from the group consisting oftrisdibromo-propylphosphate, hexabromocyclododecane and bis allyl etherof tetrabromo-bis-phenol A, wherein the flame retardant is present in anamount of from about 0.6 to about 1.2 weight percent, based on the totalweight of the formulation.

Preferably but optionally the formulation of the present inventionfurther comprises a "flame retardant synergist", i.e. one or morecompounds which increase the effectiveness of the flame retardant whenused in combination therewith. The flame retardant synergist may atleast one member selected from the group consisting of dicumyl peroxideand other organic peroxides which have a half-life of 1 hour attemperatures of from about 110° C. to about 150° C.

The polystyrene particles can also contain other additives which impartparticular properties to the expandable products, such as, antistaticagents, stabilizers, colorants, lubricants, fillers, substances whichprevent agglomeration during prefoaming, e.g. zinc stearate,melamine-formaldehyde condensates or silica, and agents for reducing thedemolding time during final foaming, e.g. glycerol esters orhydroxycarboxylic acid esters. Depending on their intended effect, theadditives may be homogeneously dispersed in the particles or be presentas a surface coating.

EXAMPLE 1 Method of Making Polymer

A mixture of 87 parts of water, 0.16 parts of sodium pyrophosphate, and0.27 parts of magnesiun sulfate heptahydrate was reacted with stirringat ambient temperature in a stainless steel pressure resistant vessel.To this mixture was added a mixture of 100 parts of styrene, 0.14 partsof benzoyl peroxide, 0.32 parts t-butylperbenzoate, 0.62 parts ofhexabromocyclododeane, and 0.21 parts of dicumyl peroxide, withstirring. The vessel was heated for at least 2 hours at a constant rateto 85° C. and then to 115° C. over 4.5 hours. Sixty-five to seventy-fiveminutes after the vessel reached 80° C., 2.9 parts of a 10% aqueoussolution of poly-n-vinylpyrrolidone was added to the reaction mixture.After an additional 100-120 minutes, a solution of 0.10 parts of chaintransfer agent in 4.7 parts of n-pentane was added to the reactionvessel. After reaching 115° C., the vessel was held at constanttemperature for 3 hours, whereupon it was cooled to ambient temperatureover 3 hours.

EXAMPLE 2

A polystyrene polymer was prepared substantially as described inExample 1. The polymer contained approximately 3.1 percent pentane(blowing agent). The resulting expandable polystyrene beads wereanalyzed according to the procedure of Example 7, and were found to havecontained polymer having a polydispersity of 1.82, a weight averagemolecular weight of 202,000, and an Mz:Mn of 2.70. The beads werescreened to 0.6-1.3 mm diameter, dried to remove surface moisture, andcoated with 0.12 weight percent of a mixture of powdered lubricants andantilumping agents commonly used in the industry as screening aids andantilumping agents. The pentane content of the beads out of thepolymerization reactor was 3.41 weight percent. However, as was typical,about 0.3 weight percent of pentane was lost during subsequentprocessing, making 3.1 weight percent the approximate pentane content atthe time of expansion.

The coated beads were expanded in a Tri Manufacturing Model 502expander. The inlet steam temperature was about 211° F., and the inletsteam flow rate was approximately 74 pounds per hour. The first-passexpansion rate was about 208 pounds per hour and the outlet density ofthe prepuff was about 1.9 pounds per cubic foot. A fluidized bed drier(as commonly used in the industry) was utilized to cool and partiallystabilize the resulting prepuff. The fluidized bed dryer was equippedwith a blower which fluidized a protion of the beads with ambient air.The prepuff was then pneumatically conveyed to storage bags and aged atambient temperature and humidity for about 3 hours. Following aging, theprepuff was expanded again in the same expander operated at the sameconditions, the result being a prepuff having a density of about 1.10lb/cu.ft., the expander operating at a throughput of about 217 poundsper hour. The resulting prepuff was again passed through the fluidizedbed dryer. After airveying again to storage bags and aging for aboutthree hours, the prepuff was transferred to a Kurtz vacuum block mold(4'×8'×34") and molded. The molding cycle consisted of presteamingvacuum to about 0.5 bar absolute pressure, followed by cross-steamingand autoclaving with steam. The resulting block was then cooled withvacuum until the foam pressure was stabilized. The pressure release timewas about 30 seconds, and the resultant block had an average of 10%fusion, 1% shrinkage (defined as actual block length shrinkage 24 hoursafter molding compared to actual mold length), 1.6% collapse (defined asactual thickness shrinkage in middle of block compared to actual moldthickness), and a bulk density of 1.10 lb/cu.ft. (defined as weight ofblock divided by actual mold volume).

EXAMPLE 3

This example illustrates the expansion and molding of relatively largepolystyrene beads containing 4.4 weight percent pentane. The effect ofinsufficient aging time prior to molding is herein demonstrated. Inaddition, a comparison is given with respect to materials containingconventional amounts of pentane.

An expandable polystyrene bead product containing an average of 4.40weight percent pentane and a bead diameter of 1.3-1.9 mm was expanded ina Kurtz KV1000 expander equipped with a Kurtz automatic density controlsystem which adjusted inlet steam flow to achieve desired prepuff outletdensity. A fluidized-bed dryer was utilized for both expansions. Thefirst-pass expansion was at a rate of 2,000 lbs/hr and a density of1.22-1.25 lb/cu.ft. After about two hours age, the prepuff was expandedagain, at a rate of 3,000 lbs/hr to a density of 0.88-0.90 lb/cu.ft. Theprepuff was then molded on a Kurtz vacuum block mold (26"×49.5"×196").No pre-steam vacuum was used. Steam was added at a pressure of about 0.6bar for about 6 seconds cross-steam followed by about 10 secondsautoclave. Vacuum was used to cool the block in the mold. After only onehour prepuff aging, the blocks were of poor quality, i.e. poorly fusedand deformed (poor dimensional stability). After 3-4 hours prepuffaging, the blocks were molded to 0.7 bar maximum foam pressure and wereof excellent quality with a total cycle time of 160-170 seconds. Typicalcycle times with normal pentane product after 24-36 hours aging were300-360 seconds.

EXAMPLE 4

This example illustrates how the use of a 4.4% pentane formulationcompares favorably with respect to the use of a conventionalformulation. Expansion and molding results, cycle time, fusion, anddimensional stability were acheived with the 4.4% pentane formulationover a conventional 6% pentane formulation.

An expandable polystyrene bead product containing an average of 4.40%pentane and a bead diameter of 0.6-1.3 mm was expanded in a Weiser VN400expander equipped with a fluidized-bed dryer. The first-pass rate wasabout 2,600 lb/hr at an outlet density of about 1.20 lb/cu.ft. Afteraging for about four hours, the prepuff was expanded again, at a rate ofabout 4,000 lb/hr and at an outlet density of about 0.79-0.88 lb/cu.ft.After about one hour of aging, the prepuff was molded in a WeiserVacuCompact block mold (196"×49"×31"). Cycle times, fusion, anddimensional stability were equal to or better than that of products ofnormal (6%) pentane content which had been aged overnight (i.e. overeight hours of aging).

EXAMPLE 5

This example illustrates, among other results, the advantageousperformance of a formulation comprising 3.6% pentane. Note the highlydesirable low aging time as well as the desirable molding cycle timewith accompanying high dimensional stability after molding.

An expandable polystyrene bead product containing an average of 3.58%pentane and a bead diameter of from 0.6-1.3 mm was expanded in a WeiserVN400 expander equipped with fluidized bed drying. The first-passexpansion rate was about 3,100 lb/hr at an outlet density of about 1.59lb/cu.ft. After aging about four hours, the prepuff was expanded again,at a rate of about 3,400 lb/h and outlet density of about 0.84-0.86lb/cu.ft. After about one hour age, the prepuff was molded on a WeiserVacuCompact block mold. Pressure-release (i.e., cooling) time was only29 seconds. All blocks were well fused and dimensionally stable.

EXAMPLE 6

This example illustrates, among other advantages, how a formulationcomprising 3.63 percent pentane permits a highly advantageous moldingcycle time, with accompanying 50% increase in productivity in themolding step, due to the lower molding cycle time. Good fusion anddimensional stability are also shown.

An expandable polystyrene bead product containing an average of 3.63%pentane and a bead diameter of from 0.6-1.3 mm was expanded in aDingledein & Herbert VA-K2000 expander. The first-pass expansion ratewas 3,000 lb/hr at an outlet density of 1.66-1.75 lb/cu.ft. After agingfor about 24 hours, the prepuff was expanded again, at a rate of about4,000 lb/hr and an outlet density of about 0.92-0.94 lb/cu.ft. Afteraging for about two hours, the prepuff was molded in a 16' TriManufacturing block mold. Well-fused, dimensionally stable blocks wereproduced at a rate of about 15 blocks per hour, as compared to 10 blocksper hour for normal pentane-content beads.

EXAMPLE 7 Molecular Weight Distribution Curve Determination

The following equipment and procedure was utilized in order to generatethe molecular weight distribution curve for polystyrene polymers. Thisprocedure was utilized to both determine the molecular weightdistribution of the polystyrene polymer of the present invention, aswell as to analyze products which are herein compared and contrastedwith the polymer and formulation of the present invention.

Chromatography Equipment and Conditions

The apparatus consisted of a Waters 6000A pump with a U6K injector, aViscotek-supplied pulse dampener, two 30 cm PLGel 5 um Mixed Bedpolystyrene columns, a Viscotek Model 100 differential visometer (DV)and a Waters R401 differential refractometer (RI). The data acquisitionand analysis hardware consisted of an IBM PC AT equipped wit 640 kb RAM,a 30 Mb fixed disk and two 5.25" floppy disk drives; a dot matrixprinter and an HP 7475A plotter. The software used was Unical Ver. 3.11(an ASYST-based package) modified to display M_(z+1) and obtained fromViscotek. The chromatographic conditions were as follows:

    ______________________________________                                        Nominal flow rate:                                                                           1.0 ml/min                                                     Solvent:       THF, high purity, non-spectro grade                            sample Injection Volume:                                                                     0.100 ml                                                       RI Detector:   Attenuation: 16x                                               Polarity:      +                                                              DV Detector:   Temperature: 31.0 ± 0.1° C.                          Full Scale Output:                                                                           50 Pa                                                          DPT Sensitivity:                                                                             0.2074                                                         Data Acquisition:                                                                            Start Time: 6 min.                                             Stop Time:     24 min.                                                        ______________________________________                                    

Analytical Procedure

The THF to be used as the mobile phase for the GPC system was filteredthrough a 0.45 um fritted filter and then degassed under an aspiratorvacuum for approximately 45 minutes. The THF and the flask were thentransferred to the GPC system and the THF maintained under a pad ofhelium. Samples were made up to a concentration of 5 mg/ml and filteredthrough a Gelman Acrodisc CR PTEE 0.45 um filter prior to injection.

Only freshly prepared solutions were used as polymer degradation wasapparent with aged solutions. All solutions were analyzed twice. The EPS(expanded polystyrene) samples were not purified by precipitation priorto dissolution and analysis, as comparison of purified and raw EPSpolymer results indicated no significant differences. To obtain accuratesolution concentrations for the unprecipitated EPS, the initial solutionconcentrations were corrected for volatiles determined by GC andcoulombmetric analysis for pentane and moisture, respectively, orgravimetrically by baking a sample for total volatiles.

To correct for fluctuations in flow rate, each chromatogram wasnormalized to the flow rate present during calibration. This wasaccomplished by calculating the ratio of the void volumes (totalexclusion volume, earliest negative peak in chromatogram) of thecalibration chromatograms to the sample chromatogram. The calibrationvoid volume was determined to be 19.72 ml. The ratio was then enteredinto the data analysis package as the corrected flow rate.

EXAMPLES 8-15

The procedure of Example was substantially followed in making apolystyrene formulation according to the present invention. This polymerformulation is identified as Example 8 in Table I (infra). The polymerformulation of Example 9 contained a blowing agent (pentane) in anamount of about 3.5 weight percent. The polymer of Example 9 wasanalyzed according to the procedure set forth in Example 7, and fromthis analysis the number average molecular weight (Mn), the weightaverage molecular weight (Mw), and the z-average molecular weight(M_(z)) were determined. From these values the polydispersity (PD) andthe Mz:Mn were calculated.

The analytical procedure of Example 7 was also performed for severalcurrent commercial products (i.e. Examples 9-14), each of whichcontained blowing agent in an amount of 5.5 weight percent to at least 7weight percent. Example 10 had blowing agent therein at a level ofapproximately 6 weight percent. From this analysis, the same molecularweight determinations were made. Table I (below) provides the results ofthe analyses for both the formulation of the present invention (Example9) as well as several commercial formulations currently available.

As can be seen from Table I, only the formulation of the presentinvention had all three identifying characteristics within the scope ofthose which are identified as pertaining to the polymer of the presentinvention. Even though each cf these polymers falls within thedefinition of the polymer utilized in the formulation of the presentinvention, none of these formulations had the amount of blowing agentrequired in the formulation of the present invention.

                  TABLE I                                                         ______________________________________                                        POLYMER CHARACTERISTICS                                                       EXAM-  M.sub.n x10E5                                                                           M.sub.w x10E5   M.sub.2 x10E5                                PLE    (g/mol)   (g/mol)   PD    (g/mol) M.sub.2 :M.sub.n                     ______________________________________                                         8     1.12      2.02      1.82  3.00    2.70                                  9     1.12      2.17      1.95  3.35    2.99                                 10     0.87      1.93      2.20  3.11    3.56                                 11     1.37      2.89      2.10  5.28    3.84                                 12     1.20      2.51      2.09  4.58    3.81                                 13     1.01      1.89      1.88  3.07    3.04                                 14     1.13      2.20      1.94  3.83    3.37                                 15     1.27      2.73      2.15  5.03    3.96                                 ______________________________________                                         [polydispersity was calculated as M.sub.w /M.sub.n, and the ratio of          M.sub.2 to M.sub.w was calculated by dividing the value obtained for Mz b     the value obtained for Mn]-                                              

EXAMPLES 16-17

These two examples illustrate the difference in both (1) totalemissions, as well as (2) emissions during aging, for expandedpolystyrene which was produced according to the process described inExample 16 illustrates the emissions from a "conventional" process whichemploys a blowing agent (pentane) in an amount of 6 weight percent.Example 17 illustrates, in contrast, the emissions from a processemploying a formulation which comprises only 3.5 weight percent pentane.For Example 16 the amount of blowing agent added during thepolymerization was approximately 6 weight percent, whereas for Example17 only about 3.5 weight percent blowing agent was added during thepolymerization. Table II (infra) provides the results of emissionsduring each of the expansion steps for each Example, for each agingperiod for each Example, and during the molding step for each Example.The bottom row of Table II provides figures for the total emissionsduring the entire process of expansion, aging and molding.

As can be seen from the figures in Table II, the polystyrene having thelow initial level of blowing agent (i.e. Example 17) exhibited a totalemissions of only from 42 to 79 percent as much as for Example 16.Furthermore, the total emissions during aging was only from about 17percent to about 48 percent for Example 17 as compared with Example 16.Accordingly, the formulation (and polymer) of the present inventionexhibit a substantial reduction in both the total emissions as well asthe emissions during aging.

                  TABLE II                                                        ______________________________________                                                       Example 16                                                                             Example 17                                            ______________________________________                                        blowing agent content                                                                          6 wt. percent                                                                            3.5 wt. percent                                   1st Pass Expansion                                                                              .76       .45                                               1st pass aging emissions                                                                       2.0        .12                                               1st pass aging time                                                                            24 hours   4 hours                                           2nd pass Expansion                                                                             N/A        .07                                               2nd pass aging emissions                                                                       N/A        .22                                               2nd pass aging time                                                                            N/A        (2 hours)                                         molding emissions                                                                              0.6-1.1     0.76%                                            Total Emissions   3.36-3.86%                                                                               1.62%                                            ______________________________________                                    

EXAMPLE 18 Method of Determining and Calculating the Degree of Branching

Branching can be determined using a Viscotek differential viscometer andrelated software according to the theory of Zimm and Stockmayer. Moreextensive and descriptive discussions of this theory, as well as relatedsubject matter, can be found in Billmeyer, F. W., Jr., Textbook ofPolymer Science, 2nd Ed., 1971, Wiley-Interscience, N.Y., N.Y.,especially pages 89-90, which is herein incorporated by reference. Usingthe Mark-Houwink constants for known linear samples of polystyrene, onecan calculate the branching frequency of the number of branches per 100monomer units. More extensive and descriptive discussions of theseconstants, as well as related subject matter, can be found in Billmeyer,F. W., Jr., Textbook of Polymer Science, 2nd Ed., 1971,Wiley-Interscience, N.Y., N.Y., especially pages 86-87, which are hereinincorporated by reference.

We claim:
 1. A polystyrene polymer exhibiting:(a) a polydispersity offrom about 1 to less than 2.0, (b) a weight average molecular weight ofgreater than about 200,000 to about 300,000, and (c) an M_(z) :M_(n) offrom about 2 to less than about 3.0, wherein the polystyrene polymer isbranched to from 0 to less than 5 weight percent.
 2. A polystyrenepolymer as described in claim 1 wherein the polymer exhibits apolydispersity of from about 1.5 to less than 2.0.
 3. A polystyrenepolymer as described in claim 1 wherein the polymer exhibits apolydispersity of from about 1.7 to about 1.98.
 4. A polystyrene polymeras described in claim 1 wherein the polymer has a weight averagemolecular weight of from greater than 200,000 to about 250,000.
 5. Apolystyrene polymer as described in claim 1 wherein the polymer has aweight average molecular weight of from greater than 200,000 to about220,000.
 6. A polystyrene polymer as described in claim 1 wherein thepolymer has an M_(z) :M_(n) of from about 2.5 to less than 3.0.
 7. Apolystyrene polymer as described in claim 1 wherein the polymer has anM_(z) :M_(n) of from about 2.7 to less than 3.0.
 8. A polystyrenepolymer as described in claim 1 wherein the polymer is ranched to from 0to less than 2 weight percent.
 9. A polystyrene polymer as described inclaim 1 wherein the polymer is ranched to from 0 to less than 1 weightpercent.
 10. A polystyrene polymer as described in claim wherein thepolystyrene is substantially homopolymeric polystyrene polymer.
 11. Apolystyrene polymer as described in claim 1 wherein the polystyrenepolymer is a substantially unsubstituted polystyrene homopolymer.
 12. Apolystyrene polymer as described in claim wherein the polystyrenepolymer is a substantially homopolymeric, unsubstituted polystyrenepolymer.
 13. A polystyrene polymer exhibiting:(a) a polydispersity offrom about 1.5 to less than 2.0, (b) a weight average molecular weightof greater than about 200,000 to about 250,000, and (c) an M_(z) :M_(n)of from about 2.5 to less than about 3.0, wherein the polystyrenepolymer is branched to from 0 to less than 2 weight percent, and whereinthe polystyrene polymer is a substantially homopolymeric, unsubstitutedpolystyrene polymer.
 14. A polystyrene polymer exhibiting:(a) apolydispersity of from about 1.7 to about 1.98, (b) a weight averagemolecular weight of greater than about 200,000 to about 220,000, and (c)an M_(z) :M_(n) of from about 2.7 to less than about 3.0. wherein thepolystyrene polymer is branched to from 0 to less than 1 weight percent,and wherein the polystyrene polymer is a substantially homopolymeric,unsubstituted polystyrene polymer.